Background: Mental retardation is defined as impaired mental capacity and ability to comply with environmental and social conditions. Chromosomal abnormalities are the most important causes of mental retardation. Carriers of balanced chromosomal translocation are phenotypically normal, although they may be at risk of infertility, recurrent miscarriage or giving birth to mentally retarded children. These abnormalities are caused because chromosomes participated in the reciprocal translocations produce quadrivalents at meiosis. These quadrivalents segregate and lead to several different meiotic outcomes, just two of which are normal or balanced.

Case Presentation: A consanguineous family with three mentally retarded daughters at the ages of 24, 18 and 10 years was referred to Comprehensive Medical Genetics Centre, Shiraz, Iran in 2015. Family history showed a case of unexplained infant death as well as a spontaneous abortion. Three survived siblings had hypotonia and severe developmental delay during infantile period. In addition, they suffer from primary amenorrhea. Two siblings have vesicoureteral reflux (VUR). Cytogenetic analysis of two patients showed 46,XX,t(6;12)(q23;q22),der(9)t(8;9)(q24;p24) with partial monosomy of chromosome 9 and partial trisomy of 8q24 segment, while the other patient had 46,XX,der(12)t(6;12)(q23;q22) with partial monosomy of 12q22qter and partial trisomy of 6q23qter segment. Their mother had two balanced chromosomal translocations (46, XX, t(6,12)(q21;q22), t(8,9)(q24;p24)).

Conclusion: The above presented case is another example for the rare occurrence of double balanced chromosomal translocations in a phenotypically normal person. Although the most important causes of mental retardation in consanguineous marriages are autosomal recessive disorders, the role of chromosomal aberrations in mental retardation in these families must not be neglected. In other words, cytogenetic studies should be performed as a first line test in either situation.